NORTH BRADDOCK (KDKA) — Two young twin brothers were killed when a fast-moving, smoky fire swept through their family home Friday afternoon in North Braddock.

Firefighters and investigators were on the scene at the two-story, single-family home at the corner of the 1200-block of Rebecca Avenue and Packer Avenue well into the night.

A contractor working in the neighborhood told investigators he saw smoke and flames coming from the wood-frame home.

He pounded on the door but got no response and called 911.

What caused the smoky, fast-moving fire has not been determined. But investigators think they know where the fire started.

“Right now, we’re looking at the level of origin being the first floor,” said Chief Alvin Henderson, of Allegheny County Emergency Services. “We’re just really starting to dig into the scene right now. Right now, all indications, it appears that the fire was probably somewhere on the first level.”

Relatives identified the children killed in the fire as 3-year-old twin brothers, Kyier and Dyier Arthur.

“They was beautiful kids,” said Darran Miller, the boy’s godfather and cousin. “They was smart, intelligent.”

A former boyfriend of the children’s mother said he heard about the fire when she called him in a panic.

“And when she called, the house was actually in flames then cause she called very hysterical and she said, ‘Oh my God.’ She’s like, ‘Ron, my house is on fire and the kids is in the house.’ So I said, ‘Slow down.’ I said, ‘What’s going on?’” said Ronald Reed, the mother’s ex-boyfriend. “Then she said, ‘I left the house for a second to go find my daughter, Mariah.’ Her 15-year-old. And she said, ‘When I came back to the house, the house was engulfed in flames.”

Officials say one of the boys was found on the first floor and the other on the second.

“I just want people to know… don’t take your eyes off your kids for two seconds because anything can happen,” Miller said. “Look at this house; it just stopped burning. They was only 3-years-old.”

One firefighter suffered a minor injury while fighting the blaze.

Allegheny County Homicide Detectives and the Allegheny County Fire Marshal are investigating the cause of the fire.